Sound reproduction devices transmitting sound information only to certain target audiences by using loudspeakers capable of providing the sound information with directivity. FIG. 6 is a schematic diagram of sound reproduction device 500 disclosed in Patent Literature 1.
Carrier wave selector 101 selects a single frequency out of plural frequencies of ultrasonic wave carrier signals, and outputs the selected frequency signal to ultrasonic wave oscillator 103. Ultrasonic wave oscillator 103 oscillates and outputs a carrier wave signal with the frequency to carrier wave modulator 105. On the other hand, reproduction signal generator 107 for reproducing audible sound outputs an audible sound signal to carrier wave modulator 105. Carrier wave modulator 105 modulates the carrier wave signal with the audible sound signal, and outputs the modulated carrier wave signal. The modulated carrier wave signal is input to ultrasonic loudspeaker 109. Ultrasonic loudspeaker 109 emits sound having directivity in response to the modulated carrier wave signal.
An operation of sound reproduction device 500 will be described below. FIG. 7A shows audible sound signal 111 reproduced by reproduction signal generator 107. FIG. 7B shows carrier wave signal 113 generated by ultrasonic wave oscillator 103. FIG. 7C shows modulated carrier wave signal 115 generated by carrier wave modulator 105. Carrier wave modulator 105 produces modulated carrier wave signal 115 by modulating carrier wave signal 113 with audible sound signal 111. In modulated carrier wave signal 115, the period of carrier wave signal 113 is changed according to amplitude of audible sound signal 111. As shown in FIG. 7C, modulated carrier wave signal 115 has a waveform having the period changes partially and having constant amplitude. Ultrasonic loudspeaker 109 has a diaphragm having a piezoelectric element attached thereto. Modulated carrier wave signal 115 input to the piezoelectric element of ultrasonic loudspeaker 109 causes the diaphragm to vibrate and generate rarefactions and compressions in the air, thereby outputting an ultrasonic wave of modulated carrier wave signal 115 to the atmosphere from ultrasonic loudspeaker 109. When this ultrasonic wave reaches ears of a user, the user can capture only compressional vibrations of the air in an audible band since the user cannot hear the compressional vibrations in an ultrasonic band. Here, the ultrasonic wave propagates with directivity of a narrow angle since modulated carrier wave signal 115 output from ultrasonic loudspeaker 109 has frequencies in the ultrasonic band. The user of sound reproduction device 500 can hence hear the audible sound only within a narrow area within which modulated carrier wave signal 115 propagates.
In sound reproduction device 500, ultrasonic loudspeaker 109 is driven with constant amplitude, as shown in FIG. 7C. If sound reproduction device 500 is used for a long period of time under such a condition, the frequency and amplitude of modulated carrier wave signal 115 may fluctuate due to heat-up of the piezoelectric element of ultrasonic loudspeaker 109 and changes in the ambient temperature. This fluctuation may change the sound pressure reproduced by sound reproduction device 500 and cause sound quality to deteriorate.